This invention relates to ground fault interrupters for personnel protection and particularly to such interrupters adapted for use in multipole applications.
In Misencik et al. U.S. Pat. No. 3,999,103, Dec. 21, 1976, is disclosed a multipole ground fault circuit breaker with interrupting capability in the event of a ground fault from any one of two or more line conductors or power poles of an alternating current electrical distribution system that includes, in addition to the line conductors, a neutral conductor that is grounded proximate the alternating current source. Features of such apparatus are also disclosed in Misencik U.S. Pat. No. 4,015,169, Mar. 29, 1977, and copending application Ser. No. 558,618, filed Mar. 14, 1975 by Zakrzewski et al., assigned to the present assignee, now abandoned. The experience to the present with such apparatus has been generally quite satisfactory. However, there is an aspect of its manner of operation that can cause undesirable operation under some circumstances.
Ground fault interrupters for personnel protection are required by industry standards to maintain a trip level that is nominally 5 milliamperes, that is the current level produced by a ground fault which causes interruption is to be in any given device equal to 5 milliamperes plus or minus 1 milliampere. Achievement of this trip level is readily within the capability of the apparatus described in the foregoing patents and application. With both poles of a two pole ground fault circuit breaker energized, which is the normally expected case, a satisfactory trip level within the standard is produced. But, with only one of the two poles energized, it has been found by experience that there is a tendency for the trip level to be reduced to a lower than desirable level. That is, the amount of current causing interruption can therefore be so low as to cause nuisance tripping of the apparatus. It is primarily with respect to the problem of providing an easy means of trip level adjustment in multipole ground fault interrupters that the present invention is directed.
The trip level is primarily determined by the characteristics of the sensing amplifier and trip circuit of the device. A given amplifier and trip circuit is subject to possible variation in performance depending on how it is biased. For example, a sense amplifier and trip circuit that has been found to be very useful is that disclosed in Engel et al U.S. Pat. No. 3,852,642, Dec. 3, 1974, which has an advantage of being operable on a half-wave rectified supply, as well as a full-wave rectified supply. In the latter case, encountered in multi-pole applications, the off-time of the amplifier is reduced to a minimum.
It is found that if a unit is designed and made to have a trip level of 5.0 ma. on a full-wave supply, that same unit will exhibit a reduced trip level on a half-wave supply. The amplifier for a 2-pole GF breaker is set at 5.0 ma. assuming both poles are on. If the unusual occurs in the use of such a unit and the user forces the handle for one pole on and the other pole off, then the amplifier is seeing a half-wave rectified supply that reduces the trip level, typically about 0.5 ma. That is, if a unit is tested and found to operate at a trip level in the low part of the acceptable range, say 4.2 ma. with both poles energized, it could have a trip level of 3.7 ma., outside the desired range, with only one pole energized. This invention is aimed to correct that situation.
In part, the present invention utilizes principles discussed in Misenick et al U.S. Pat. No. 4,056,837, Nov. 1, 1977 which shows how a resistive circuit branch connected between line and neutral conductors on opposite sides of a differential current transformer of a GFI influences the effective trip level of the device.
Another situation has been found in multi-pole GFI applications which is to be improved. Where the electrical system is three phase, the phase relation of the line voltages is such as to reduce the trip level. So if a given unit is made to exhibit a 5.0 ma. trip level on single phase operation from two poles, its trip level is reduced if applied to a three phase system, even if all poles of the three phase system are on. An aspect of the invention is therefore to extend the utility of GFI's for permitting application of a single unit to either single or multi-phase applications.